[Paper Review] Indications for Cluster Melting from Forward-Backward Charge Fluctuations at RHIC Energies

M. Abdel-Aziz, Marcus Bleicher|ArXiv.org|May 26, 2006

High-Energy Particle Collisions Research3 citations

TL;DR

This paper proposes a model linking forward-backward charge fluctuations in Au+Au collisions at RHIC to cluster decay dynamics, showing that decreasing cluster multiplicity (k) and short-range correlation length (λ_short) in central collisions indicate partial melting of clusters due to interactions in a hot, dense medium. The model predicts k ≈ 1.9 and λ_short ≈ 0.3 at 0%-3% centrality, suggesting near-complete cluster melting in the most central collisions.

ABSTRACT

We study forward-backward charge fluctuations to probe the correlations among produced particles in ultra relativistic heavy ion collisions. We develop a model that describes the forward-backward dynamical fluctuations and apply it to interpret the recent PHOBOS data. Within the present model, the dynamical fluctuations are related to the particle production mechanism via cluster decay and to long range correlations between the forward and backward rapidity hemispheres. We argue that with a tight centrality cut, PHOBOS may see a strong decrease of the dynamical fluctuations. Within the present model, this deterioration of the correlation among the produced hadrons can be interpreted as a sign for the production of a hot, dense and interacting medium.

Motivation & Objective

• To investigate the particle production mechanism in ultra-relativistic heavy ion collisions, particularly whether clusters (e.g., hadronic resonances, partonic strings, or QGP droplets) survive or melt in dense matter.
• To determine whether forward-backward charge fluctuations measured by PHOBOS can serve as a probe of cluster decay and medium effects in Au+Au collisions at √sNN = 200 GeV.
• To quantify the fraction (f) of particles produced via cluster decay and the effective cluster multiplicity (k), and to assess how these vary with collision centrality.
• To test the hypothesis that cluster melting in central collisions signals the formation of a hot, dense, interacting medium such as the quark-gluon plasma.

Proposed method

• The model decomposes the variance of the forward-backward correlation variable C into contributions from short-range (SR) and long-range (LR) correlations, weighted by the fraction f of particles from cluster decay.
• The short-range fluctuation term σ²_SR is modeled as k[1 − exp(−Δη/λ_short)], where k is the effective cluster multiplicity and λ_short is the short-range rapidity correlation length.
• The model is applied to PHOBOS data on forward-backward charge fluctuations in Au+Au collisions across centrality bins from 0%-20% to 40%-60%.
• The model parameters f and k are extracted by fitting the measured σ²_C to the theoretical expression σ²_C = fσ²_SR + (1−f)σ²_LR, with σ²_LR representing long-range correlations.
• Linear extrapolation of k and λ_short to 0%-3% centrality is performed to estimate the degree of cluster melting in the most central collisions.
• The analysis focuses on the soft momentum region (pt < 2 GeV) to minimize jet-induced artifacts that could mimic clustering effects.

Experimental results

Research questions

• RQ1 Does the effective cluster multiplicity k decrease in central Au+Au collisions compared to peripheral ones, indicating cluster melting?
• RQ2 How does the short-range rapidity correlation length λ_short evolve with centrality, and what does it imply about cluster survival?
• RQ3 Can forward-backward charge fluctuations be used to distinguish between cluster decay and independent particle emission in heavy-ion collisions?
• RQ4 To what extent do interactions with a dense medium suppress cluster decay signatures, and can this be quantified?
• RQ5 What is the predicted behavior of k and λ_short in the most central collisions (0%-3%), and does it approach the limit of complete cluster melting?

Key findings

• The effective cluster multiplicity k decreases from 2.7 in 40%-60% peripheral collisions to 2.2 in 0%-20% central collisions, indicating reduced cluster size or survival in central collisions.
• The short-range rapidity correlation length λ_short decreases with increasing centrality, suggesting reduced spatial and rapidity correlation range of decay products.
• Linear extrapolation to 0%-3% centrality predicts k ≈ 1.9 and λ_short ≈ 0.3, indicating partial melting of clusters in the most central collisions.
• The fraction f of particles produced via cluster decay also decreases toward central collisions, implying a reduction in correlated particle emission.
• The observed decrease in k and λ_short is interpreted as evidence of medium-induced cluster melting, possibly signaling the formation of a hot, dense, interacting medium such as the quark-gluon plasma.
• The results suggest that in the most central collisions, cluster decay may be suppressed to the point of near-complete melting, with k approaching 1 and independent particle emission dominating.